How to compare with null value in Oracle?

In Oracle, comparing with null values can sometimes be a tricky task. Null represents the absence of a value, making it unique and often unpredictable in its behavior. This article will guide you through different approaches to compare null values in Oracle and provide clarity on this often confusing concept.

How to Compare with Null Value in Oracle?

In Oracle, you cannot compare null values using the standard comparison operators like =, <, or >. Instead, you should use the special equality operator IS NULL. This operator evaluates whether a given expression or column is null or not.

Here’s an example to illustrate its usage:

“`sql
SELECT *
FROM employees
WHERE salary IS NULL;
“`

In the example above, the query selects all employees whose salary is null. The IS NULL operator comes in handy when you want to filter or identify null values within your database.

Now let’s explore some frequently asked questions related to comparing null values in Oracle:

1. Can I use the equality operator (=) to compare null values in Oracle?

No, comparing null values using = will always yield a null result, not true or false. Hence, IS NULL is the appropriate operator to use for null comparisons.

2. How do I compare if a column is not null in Oracle?

To check if a column is not null, you can use the IS NOT NULL operator. It returns true if the column holds a non-null value.

3. Can I use NULL in a WHERE clause to match null records, similar to the IS NULL operator?

No, using NULL directly in a WHERE clause will not yield the desired result. You should always use IS NULL instead.

4. What happens if I compare a value with null using the != operator?

The != operator, also known as the inequality operator, returns unknown when comparing with null. It should not be used for null comparisons.

5. How can I compare two columns, both of which can be null?

To compare two columns that can be null, you can use the special operator IS NOT DISTINCT FROM. This operator considers null values as equivalent during the comparison.

6. What is the result of arithmetic operations involving null values?

Any arithmetic operation involving a null value will result in null. For example, adding or subtracting null with any other value will result in null.

7. Are null values indexed in Oracle?

Null values are not indexed in Oracle’s standard B-tree indexes. However, Oracle provides a bitmap index that can handle null values efficiently.

8. Can I use the CASE statement to compare with null values?

Yes, the CASE statement can be used to handle different conditions, including null values. You can specify conditions for null values explicitly in your CASE statement.

9. How does the NVL function help in comparing nulls?

The NVL function allows you to substitute a null value with another value of your choice. It can be used in comparisons to treat null values as a specific value for more accurate results.

10. Are null values treated differently when using the BETWEEN operator?

No, null values are not treated differently when using the BETWEEN operator. If a column has a null value, it will not be included in the result.

11. Can I compare a null value with an empty string (”)?

Yes, you can compare a null value with an empty string using the appropriate operator. However, keep in mind that null values and empty strings are not equivalent in Oracle.

12. How can I find rows with null values in multiple columns simultaneously?

To find rows with null values in multiple columns, you can combine multiple IS NULL conditions using the logical AND operator.

In conclusion, comparing null values in Oracle requires special attention. Using the IS NULL and IS NOT NULL operators appropriately can help you handle null values effectively in your queries. Remember that null behaves differently from regular values, and understanding how to compare with null will assist you in accurately querying your database.

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